The immune response to transplanted kidneys between strains of congenic mice will be compared with that of transplanted sarcomas (S-702) in the same strains. In both cases the transplants survive in the face of an H-2 determined difference between the strains concerned (B10.D2 donors and B6AF1 recipients). We will explore further the importance of suppressor cells in transplant survival by in vitro tests of cells from recipients and will also determine the effects of specific suppressor T cells from cloned cell lines or "suppressor factor" from culture supernatants on the survival of transplanted tissues.